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Mercury Carburetors Leaking

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riocasa

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Two of the 3 carburetors are leaking gas on my 40ELPT, 4S, outboard (2000 year) motor. The leaks are between the float bowl and body, next to the plunger. I have rebuilt these twice, and used a new gasket kit the last time. The motor has very few hours, and looks brand new, but, I can't synchronize until I stop the leaks. I checked the float level, appears OK. Does anyone have any ideas, or do I assume the carbs are warped, or somehow shot??
Thanks for any help.
Riocasa
 
Had that same problem lately.

It's hard to do, but you need to determine it the float bowl gasket is leaking, or if it's flooding.

Remove the carbs (again!) and rig up the gas hose right to them, one at a time. Pump the bulb until the leak happens and study where it's coming from. If it's that damn gasket--and I had a new one leak badly on me--you might want to reuse the old one. (Fastjeff's Law: "Never throw anything away!") If you round-filed the old one, then you'll have to improvise.

If you're sure it's flooding, try lowering the float level a 1/16 of an inch at a time until it stops. Contrary to popular opinion, the float level can be set WAY lower without effecting performance. I used to run the Holley carb on my autocross car a full 1/8 " lower than "correct" to clean up its performance on bumpy tracks. The 1/8 " lowering had NO effect on output whatsoever.

Jeff (with 50 years of carb experience)
 
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Jeff, thanks much! I'm now sure the leaks are coming from in front of the gasket from a hole with a brass plug with a small gap on either side (vent?); points to a stuck float (even with a new needle). When I first sprayed carb cleaner in the plunger hole, it flowed out between these gaps, but, after more cleaning, no more leakage around the plug. What do you think of using WD40 around the float pin (the float appears in good shape)? Riocasa
 
I just discovered that the top carb, float to housing gasket has an "O-ring" attached to it, that surrounds the brass plug (where the leaks are occurring). Neither of the other two has this O-ring attached, and they are leaking (including the new kit I just bought). I cannot figure out why these carbs have not been leaking before, since I used the boat last year. Nobody at Mercury Marine or the Parts stores can help me understand which is normal, and I am confused re the brass plug, and what might happen if I seal it with an O-ring (if I can find one that size!).
Riocasa
 
To complete this thread, I located the proper gasket through Boats.net. It is important to talk to them since for every motor there are dozens of Models, and you must select the right one for the kit. Unfortunately, I got the wrong kit which did not have the small O-ring attached to the plunger plug, hence, the leaks. I solved for now by inserting a small, unattached O-ring, and the leaks stopped. The kits cost $40+ (must buy entire repair kit for one gasket), so an expensive mistake. I will now get a professional carb sync, and in the future, will not leave "old gas" in the carbs after use!
Riocasa
 
Check your float it might have fuel leaking into it which will weigh the float down giving it to much pressure inside the carburetor fuel bowl
 
fastjeff said:
Had that same problem lately.

It's hard to do, but you need to determine it the float bowl gasket is leaking, or if it's flooding.

Remove the carbs (again!) and rig up the gas hose right to them, one at a time. Pump the bulb until the leak happens and study where it's coming from. If it's that damn gasket--and I had a new one leak badly on me--you might want to reuse the old one. (Fastjeff's Law: "Never throw anything away!") If you round-filed the old one, then you'll have to improvise.

If you're sure it's flooding, try lowering the float level a 1/16 of an inch at a time until it stops. Contrary to popular opinion, the float level can be set WAY lower without effecting performance. I used to run the Holley carb on my autocross car a full 1/8 " lower than "correct" to clean up its performance on bumpy tracks. The 1/8 " lowering had NO effect on output whatsoever.

Jeff (with 50 years of carb experience)
Click to expand...

Hello, found this old thread...I'm having the same problem! Not used to working on tiny carbs like this but can't get these 77 Merc 500 carbs to stop leaking. Never had this problem before. When you mention lower float height, you mean lower it when it's upside down or right side up?It's not a major leak but I'm stumped, have replaced floats, gaskets twice, still leaking from bottom plug or bowl. Thought I finally had it, pumped up firm last time, I waited a min pumped hard, and leak began again...
 
What is that white gunk, silicone? I think your problem is you're missing gaskets in '33"77carb1.jpg
If you want to set the float level follow the second page instructions.77carb2.jpg
 
I'm going to order kits and try it again with new gaskets... I have triple checked float levels per the manual, but am thinking of also trying a lower setting, as recommended by fellow that posted before.

Also, don't mind that throttle coupler Lol. I snapped it removing the carbs so plastic welded it together and put adhesive on it. Want to make sure this thing works right before replacing anything:)
 
Just ordered kits to try again, but I think I may have finally found my problem. Watched a YouTube video where they rebuild my exact carbs, I think I have the main gasket that attaches the carbs to block (flange gaskets) backwards and/or flipped, so the vent hole is not lining up with carb and making an overflow situation. I hope I found the problem!
 
Does it leak when it's at rest? How hard are you pumping the primer bulb? Any carb will leak past the float valve if there is excessive pressure.
 
Does not leak at rest, or when it's running from what I can see. I pump it up until it's firm and just then when bulb is firm, after I hear bowls fill up, usually begins to leak, mostly from the bottom plug. Another time it leaked from side of bowl, and one time fuel even shot out of front of carb. It's been a journey to fix these.
 
I am stumped. Float levels look perfect, needles and floats are new, no gunk in the seats. Going to rebuild again with new kits with the manual at my side and triple check everything.
 
Your primer bulb is providing too much pressure perhaps. When did the problem first start? Either that or your floats are soaking in fuel......too heavy......"fuel logging". You said you put new ones in?
 
Motor was sitting for years so after compression check, started with carb rebuild. I think I will try a new fuel line and primer bulb. I'm using a 3/8" fuel line. One odd thing is I do hear a lot of "air sucking" when I prime the bulb, which is odd. I did rebuild fuel pump/carb with Oem Merc kits.
 
You must be building too much pressure or introducing small particles of debris. That would create a seating problem for the needles. I have seen this before. It often occurs on sunny days with poor tank venting.
 
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Travis......3/8 line is too big..use the 5/16 line, loose the hose clamps and use fuel ties. This wont solve the flooding but will prevent fuel/air leaks
 
Bob in Tennessee said:
I have the exact same engine. Did you solve your leakage?
Is your engine hard to start? Mine takes maybe 15 spins.
Click to expand...

Yes. The manual's recommended float height of 1/4" was too low for mine...I set them higher, probably around 5/16" to stop the flooding. Leakiest carbs ever. With that being said, you still need to be careful priming these little devils...the only thing stopping a waterfall is the doinky little weak spring attached to the inlet needle.
 
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